A. Field of Inventions
This invention relates to thermoplastic elastomer compositions comprising blends of elastomers, e.g., rubber, and thermoplastic resins, the rubber being at least partially cured. In particular, the invention is directed to such thermoplastic elastomers having a Shore A hardness of less than or equal to 35.
B. Description of Related Art
Thermoplastic elastomer compositions, e.g., blends, having a cured rubber content are well known in the art, see, for example, U.S. Pat. No. 4,130,535, where a cured, e.g., vulcanized) thermoplastic elastomer, also referred to herein as a thermoplastic vulcanizate, (“TPV”) is made from a polyolefin resin and an olefin rubber. The at least partially cured state of the thermoplastic elastomer is obtainable by subjecting the blend of uncured rubber and thermoplastic resin to a curing process. This process can be static or dynamic and be done by the use of known curing agents, like peroxides or phenolic resins.
A dynamically vulcanized polyolefinic thermoplastic elastomer typically consists of 1-5 micron sized crosslinked rubber particles in a continuous semi-crystalline polyolefin matrix, where the term “semicrystalline” as used herein broadly refers to materials that have crystallinity by DSC of at least 5% or above, but preferably at least 25% or above, as contrasted with materials that are “amorphous,” which as used herein broadly refers to materials that have crystallinity by DSC that is less than 5%. TPV composition processability, and physical properties (such as tensile strength) within the elastomer service temperature range is considered to be due to the continuous semi-crystalline polyolefin matrix. The matrix is the relatively “hard” phase as opposed to the rubber “soft” phase. The hardness of the TPV composition depends on the amount of soft and hard phase present. Reducing the “hard” semi-crystalline polyolefin phase would increase TPV softness at the expense of decreased product processability. A lower amount of the matrix, or, if the matrix is insufficient to provide a continuous phase, can result in TPV rubber particle agglomeration which would manifest itself as a “powdery” instead of a “continuous” thermoplastic on processing, for example, by melt extrusion of the product in compounding or finishing.
Physical properties would also be lowered due to insufficient matrix “glue” that holds the crosslinked rubber particles together. Thus, desirable soft TPV compositions are not readily produced. Moreover, increased TPV rubber content alone will not afford soft TPV compositions; added process oil is necessary. TPV compositions containing excess oil will mainly swell the rubber phase and therefore reduce plastic phase volume, which will result in the already mentioned disadvantages. Also, low molecular weight rubber and/or plastic molecules that are unattached to the TPV network can render the product “sticky” in the presence of oil. Unattached plastic molecules are those not incorporated into the polyolefin crystallites and unattached rubber molecules are those that are not bound to the crosslinked rubber network. Thus, commercially viable TPV compositions with hardness below 35 Shore A are difficult to produce.
Crosslinking of the rubber phase increases TPV hardness, but TPV compositions containing a large amount of uncrosslinked rubber do not provide commercially viable soft products. The ethylene content in commercially available EPDM rubber generally varies from about 50 weight % to about 70 weight %. The incorporation of uncrosslinkable EP rubber into a TPV composition or the addition of EPDM rubber to a preformed TPV composition will not yield a suitable soft product. The uncrosslinked rubber would be compatible with the crosslinked rubber phase, and will tend to be included into this phase, and thus the previously mentioned drawbacks of a high rubber content TPV composition will not be overcome. If an excessive amount of uncrosslinked rubber is present, or if all the rubber in the polyolefinic thermoplastic elastomer is uncrosslinked, then this “soft” product would suffer the processing and property disadvantages of a continuous rubber phase as opposed to those of the desirable continuous plastic phase for the thermoplastic elastomer product.
Thermoplastic elastomers targeted to have a hardness of less than 35 Shore A are described in EP-B-0 892 831. This patent describes a blend of rubber and thermoplastic resin, the rubber being at least partially cured, comprising a) the thermoplastic resin, b) an uncured amorphous poly-α-olefin or an ethylene and C3-20 α-olefin amorphous copolymer and c) an at least partially cured rubber, wherein the weight ratio of b) to a) plus b) is 15-75 wt. %, and where weight ratio of c) to a) plus b) plus c) is 25-75 wt. %. Additionally, “soft polyolefins” are addressed in the article “VERSATILE NEW SOFT POLYOLEFIN FOR COMPOUNDING WITH OTHER SOFT THERMOPLASTICS RESINS OR AS A TPV BASE RESIN”, L. Struzik, et al. (ANTEC 2003). This soft polyolefin is described as an in-situ polyolefin that has high rubber content prepared by selection of catalyst and process technology. The soft polyolefin is taught for use where elastomeric properties are sought by either blending with other polyolefins or by a process of dynamic vulcanization of the rubber content in the soft polyolefin. The examples illustrate compositions having Share A hardness at or above about 60.